Method of making a gas laser

ABSTRACT

A gas laser apparatus includes a ceramic wave guide having a bore extending therethrough and a pair of slots formed in the exterior thereof with electrodes formed in the slots. An optical assembly is mounted on each end of the waveguide and includes a mirror which is attached to a metal ring in the optical assembly where they harden a fused glass frit material. A plurality of invar metal rods connects the optical assemblies and has adjustment screws on each end of each rod for tilting a portion of the optical assembly on a metal flexure member to align the mirrors. The tubes and rods in the optical assemblies holding each mirror have temperature compensating materials and sizes to maintain the mirror in alignment, and separation over wide ambient and operating temperatures. 
     A process of making a gas laser includes the steps of selecting an optical mirror blank, making a metal mirror blank support ring having an opening therethrough for mounting the selected mirror blank therein, applying a glass paste material between the mirror blank and the mirror support ring, heating the mirror blank and the mirror blank support ring and glass paste material to a temperature in excess of 400 degrees centigrade to seal the mirror blank to the metal mirror blank support ring with a glass frit seal, and then attaching the support ring to a laser optical assembly on each end of a laser assembly.

This is a division of application Ser. No. 881,097 filed July 2, 1986, US. Pat. No. 4,713,825.

BACKGROUND OF THE INVENTION

The present invention relates to the laser apparatus and a method ofmaking the laser apparatus and especially to a gas Waveguide laserapparatus having hard sealed optical to metal seals and temperaturecompensation.

In the past, a wide variety of gas lasers have been provided. Many ofthese are CO₂ Waveguide lasers which fall within the same classificationas the present laser apparatus. Typically, a Waveguide laser has a waveguide which may be made of a ceramic material having a bore extendingtherethrough and filled with a gas such as CO₂. Electrodes are placedadjacent to a wave guide tube for pumping the gas located in theWaveguide tube. Each end of the tube generally has a mirror mountedthereto, one of them with substantially total reflection and the othermirror having a partial reflecting surface to allow the escape of laserenergy.

The present invention is directed towards a wave guide laser as well asto a method of making a wave guide laser, utilizing a ceramic Waveguidebut having a circulating gas ballast tank and electric pump to circulatethe CO₂ gas brom the ballast tank through the wave guide bore, andincludes special optical assemblies and temperature compensating design,as well as a method of making a laser apparatus in accordance with theinvention.

Prior art lasers can be seen in the Newman Waveguide Laser Pat. No.4,381,564, having a capacitively of coupled discharge which includes awave guide having a bore therethrough, along with a pair of high voltageelectrodes driven by high voltage supply. An optical assembly is mountedto each end of the wave guide for mounting mirrors at each end of thebore of the Waveguide. A tube connects each end of the wave guideassembly to connect a gas reservoir and a circulator to allow the gaswithin the bore to be replenished and recirculated. The Noble et al.,Pat. No. 4,065,370 is for the ion plating of a thin metallic strip to aflashlamp for triggering the flashlamp. Prior art seals or mounting foroptics can be seen in the prior Holtz Pat. No. 3,599,112 and in theKnowles Pat. No. 3,978,425. In the Ljung et al., Pat. No. 4,153,317, anindium seal for gas lasers is illustrated. Indium is a common prior arttechnique for attaching laser optics to optical assemblies, whileanother common prior art uses an epoxy adhesive to attach the lasermirror to the laser assembly. The Ljung Pat. also suggests the passivealignment of the optics prior to the mounting and the sealing of theoptics by optical contact, epoxy, or glass frit, then evacuating andfilling the laser. Fine alignment is accomplished in this laser bydeforming the support of one or both mirrors by using a metal tube witha neck-down section using set screws to accomplish the adjustment. Otherglass to metal seals can be seen in the Knowles Pat. No. 3,978,425 forlaser components and fabrication methods. This patent has a metal casingin which the optical elements are attached to a kovar gridwork and hasinterconnected annular portions placed in a mold and optical glasspellets introduced onto each ring so that the application of heat meltsthe glass and forms the glass directly onto the kovar rings. Alignmentof the optical elements is by displacing the metal casing slightly. InU.S. Pat. No. 4,393,506 to Laakmann et al., a method is disclosed formanufacturing a sealed-off RF excited CO₂ laser with a longer operatinglife. The present invention is also directed towards a wave guide laserwhich has a longer shelf and operating life. A number of factors havebeen identified as the principle causes of limiting the operating lifeof a sealed-off CO₂ laser. Sealed-off lasers are utilized because theydo not require auxiliary gas cylinders and vacuum pumps and so that theycan be more readily portable. However, one of the problems associatedwith sealed-off CO₂ lasers is that of maintaining stable long-termoperation despite factors which tend to destabilize the gas chemistry,such as CO₂ disassociation.

There are a large number of prior art patents which deal with eithertemperature compensation for maintaining the alignment of the optics orwith specific alignment techniques for the optics. Typical prior artpatents of this nature can be seen in the Singleton U.S. Pat. Nos.4,342,117; Marlett et al., 4,224,579; the Smars 3,671,883; Sepp et al.,4,457,001; the Hamerdinger et al., 4,149,779 and in the Barnaby3,605,036. The Singleton Pat. teaches a mirror mounting arrangement fora gas laser which connects the mirror optics to a flanged cup which isthen connected to another flanged cup attached to the body using anannular spacer member between the flanged cups and a sealing techniquethat does not require epoxy resin or indium seals.

In contrast to the prior art, the present invention provides for aWaveguide assembly having a gas ballast tank and a permanent magnetcirculating pump for circulating the CO₂ through a ceramic wave guideand has alignment rods selected and supporting the optical assemblies oneach end of the Waveguide to match the temperature expansion of the waveguide while allowing the alignment of the optical mirrors. The opticalmirrors are mounted to an expansion tube expanding in the oppositedirection from the wave guide and alignment bars and is selected tonegate the expansion of the Waveguide and alignment rods to maintain thelaser mirrors in alignment. The method of making the laser allows for awindow blank to be inserted into a window ring and attached with a glassfrit seal at 400 degrees centigrade in nitrogen and to attach this tothe optical assemblies. The optical assemblies can then be baked at ahigh temperature for long periods of time for removing moisture andimpurities therefrom prior to filling with the CO₂ gas.

SUMMARY OF THE INVENTION

A CO₂ Waveguide laser apparatus is provided which uses a ceramicWaveguide made of alumina, having a bore extending therethrough and apair of slots cut into the exterior thereof for forming the electrodestherein. An electrode is formed in each of the ceramic Waveguide slotsand has an electrode post extending therefrom. An optical assembly forholding a mirror on each end of the Waveguide is attached to each end ofthe ceramic Waveguide with the mirror being attached to a metal ring inthe optical assembly where they harden the fuse glass frit material. Aplurality of invar metal rods are connected between the opticalassemblies. The metal rods are connected to kovar rings on the mountingcup for the optical assemblies, and each has an adjustment screwextending from a second kovar ring into the metal rod with two threadedportions for adjusting part of the optical assembly relative to themetal rods for aligning the mirrors. Adjustment of the threaded rodsbend an annular kovar flexure member connecting the Waveguide connectingcup to the rest of the optical assembly. The optical assembly includes akovar metal cylinder for attaching the metal ring having the mirrormounted thereto. The invar support and alignment rods are matched to theexpansion rate of the Waveguide which expands together in one directionwith the increase in temperature in the assembly. While the kovarcylinder holding the metal ring and mirror has an expansion rateopposite to and matching the expansion of the wave guide and rods formaintaining the alignment of the mirrors. A gas ballast tank connectsgas to each end of the Waveguide and circulates the CO₂ gas between theballast tank and the Waveguide with a permanent magnet pump.

The process for making the laser includes the steps of selecting theoptical window blank of zinc selenide, then making a metal window blanksupport ring of carpenter alloy having an opening therein and mountingthe zinc selenide mirror blank therein, then applying a glass pastematerial between the mirror blank and the support ring and heating thering mirror blank and glass paste to a temperature of 450 degrees in anitrogen atmosphere to fuse the glass frit and seal the glass blank tothe metal ring. The carpenter metal window blank support ring is thenwelded to a laser Waveguide has a metallized end portion for attachingthe optical assembly support cups thereto. The wave guide assembly hasclean fired to 1,000 degrees centigrade for welding the system togetherand the body assembled by brazing to over 1,000 degrees centigrade,followed by the heli-arc welding of the optical assemblies to theWaveguide assembly. Leak tests are then performed and the entire laserassembly is vacuum baked at 300 degrees centigrade for forty-eight hoursand is then back filled with CO₂ and pinched off with a pinch-off tube.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention will beapparent from the written description and the drawings in which:

FIG. 1 is a cutaway side elevation of a gas laser in accordance with thepresent invention;

FIG. 2 is an end elevation of the laser in accordance with FIG. 1;

FIG. 3 is an end elevation of the laser in accordance with FIG. 1;

FIG. 4 is a sectional view taken on line 4--4 of FIG. 1;

FIG. 5 is a sectional view in accordance with FIG. 4 showing analternate embodiment electrode post; and

FIG. 6 is a flow diagram of the process of making the laser inaccordance with FIGS. 1 through 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings and especially to FIGS. 1 through 4, a CO₂Waveguide laser 10 is illustrated having a ceramic Waveguide 11 whichmay be made of an alumina and may have a bore 73 extending therethroughas seen in FIG. 4. The Waveguide 11 has an optical assembly 12 attachedto one end and optical assembly 13 attached to the other end thereof. Astainless steel gas ballast tank 14 is also attached to one side of thelaser for holding extra CO₂ gas for the laser and is driven by apermanent magnet pump 15 to circulate the gas from the tank through thebore in the Waveguide 11. The optical assemblies 12 and 13 are supportedby invar metal stabilizing rods 16 of which three are shown on thepresent laser. The optical assemblies 12 and 13 are connected tometallized surfaces 17 on each end of the wave guide 11 which may be aband of molybdebum and maganese with a nickel plate so that opticalassembly support cups 23 of assembly 13 and 25 of assembly 12 can bebrazed at 24 to the metallized band 17 at each end of the Waveguide 11.Cups can be brazed with a gold-copper alloy at 1,040 degrees centigrade.A slot 18 has been cut into the outside along most of the Waveguide 11on two sides thereof and has had the bottom metallized 20 with a copperelectrode 21 brazed thereto. An electrode post or connector 22 isattached to each of the elongated electrodes 21 for connecting a sourceof power such as a high voltage placed between the two electrodes 21.One electrode is an anode and the other a cathode and is used in pumpingthe laser gas. The metal supporting cups 25 are made of kovar, an alloyof nickel and iron, and have a kovar ring 26 extending therearound forattaching the rods 16 thereto on each end for stabilizing the opticalassemblies between each other. Each rod 16 has each end thereof, havinga threaded bore 27 extending thereinto for an external threadedadjustment portion 28 to be threaded into and attached to the adjustmentrod 30 which in turn has another externally threaded portion 32 ridingin a threaded bore 31 through bar support plates 34. A differentialscrew head 33 is rotated to rotate the externally threaded members 32and 28 in their threaded bores to make adjustments in the laserassembly. Plates 34 are held with screws 35 to the annular kovar member42, which in turn, is welded to an expansion kovar flange 37. An annularbellows flexure member 36 is also made of kovar and connects the plate25 to the annular kovar ring 42, while the kovar ring 42 is connected tothe expansion kovar sleeve 37. A carpenters metal plate 38 has the glassblank with a mirrored surface 40 attached thereto. Carpenters metal isan alloy of nickel and iron and is selected for its expansion rate inaccordance with the zinc selenide window blank 40. The glass blank 40attached to the carpenter metal mirror support ring 38 using a silicasealed glass paste at their abutting surfaces 41 which is fused into aglass frit material having an expansion to match the zinc selenide glass40. The three elements are heated to 440 degrees centigrade in anitrogen atmosphere to attach the mirror optics 40 to the metal mirrorsupport plate 38. The metal mirror support plate is then welded to thekovar sleeve 37 to completely seal the CO₂ gas within the Waveguide borand end optics and is connected through copper tubing to the ballasttank 14.

In operation, rotating the differential screw head 33 rotates thethreaded portions 32 and 28 to rotate the plates 34 along with the kovarring 42 having the expansion tube 37 attached thereto while bending themetal flexure rings 36 to thereby adjust the position of the mirror 40.Adjusting the three different differential screws 33, spaced equallyapart, will adjust the mirror 40 to any position desired to provide themaximum output from the laser. The optical assembly is connected at oneend through a copper tube 43 and an L-connector 61 connected to theoptical assembly 12 and through another L-connector 44 connected to theballast tank 14, so that the flow of CO₂ gas can go between the opticalassembly 12 and the ballast tank 14. A similar copper tube 45 has aT-connector 46 on one side and is connected to copper tube 65 connectedto another connector 60 which is connected to one end of the ballasttank 14. The T-connector has a pinch-off tube 47 which is used forevacuating and filling the laser and ballast tank 14 and which can bepinched off at 50. The copper tube 45 is connected through theL-connector 48 to the optical assembly 13. The CO₂ gas ballast tank 14is connected to the pump 15 through a housing portion 51 and may bewelded thereto and has an electric motor 52 for operating the pump 15.The electric motor 52 is attached to the housing 63 of the pump. Thepump includes an impeller 54 along with a spacer 55 formed in thehousing 51 and has a magnet 56 connected to a retainer 57 rotated by themotor 52. The motor 52 in turn rotates the magnet on the outside througha non-magnetic housing portion to drive an gold-plated iron impeller 54.This type of pump avoids any shaft seals and allows a complete sealedhousing between the pump motor 52 and the driving impeller 54 locatedinside a separate housing connected to a ballast tank 14 for circulatingthe gas between the tank 14 and the laser wave guide 11. Threadedfasteners 58 allow the motor and pump to be attached together throughthe support housing portion 62.

The optical assembly 13 can be seen as having the same elements as theoptical assembly 12 including support plates 66 having the differentialscrew head 67 protruding therethrough and extending into the invar rods16 at the opposite end of the invar rods 16 and a threaded fastener 68for attaching the plates to the kovar rings in the same manner asalready explained for the optical assembly 12. The optical assembly 13supports and optical mirror 70 in the same manner as that of assembly 13which may be a zinc selenide window. One of the optical assemblies has aone hundred percent mirror coat while the other has a ninety-fivepercent partial coating thereon to allow the escape of energy at one endwhile allowing the continuous reflection from both ends.

It should be noted that the invar rods 16 has been chosen to have verysmall amount of expansion due to the increase of temperature as thelaser heats up. At the same time, the kovar expansion sleeve 37 has beenchosen to have a size and a material that expands at almost theidentical rate for its temperature increase as the expansion of theinvar stabilizing rod 16 but in the opposite direction, so that whilethe ceramic wave guide 11 and invar stabilizing rod 16 are expanding inone direction due to the increase of temperature from the laser action,the cylinder 37 is expanding at almost the same rate in the oppositedirection to thereby maintain the mirrors 40 and 70 in substantialidentical alignment during the heat expansion of the laser, inaccordance with the initial alignment of the laser which has been madethrough the differential screws 33, bending the flexure plate 36.

The expansion is in accordance with the following therma expansioncompensation chart.

THERMAL EXPANSION COMPENSATION

    a (INVAR)=0.9×10.sup.-6 / F

    a (KOVAR)=3.5×10.sup.-6 / F

    a (ALUMINA)=3.5×10.sup.-6 / F

    FOR T+50 F

    L (CERAMIC W/G)=0.0018 INS.

    L.sub.1 (INVAR ROD)=+0.0003 INS.

    L.sub.2 (2-KOVAR CUPS)=0.00029 INS.

    NET SEPN. OF MIRRORS=0.00001 INS.

In addition, the thermal expansion of the zinc selenide window mirrorblank 40 has been chosen to match the carpenter metal mirror supportingdisk 38 and the glass frit sealing area 41 has been chosen to be of asimilar expansion rate so as to prevent the mirror 40 and 70 frombreaking loose from the seal due to the expansion and contraction whenthe laser heats up and cools down. The zinc selenide window blank ismounted to a carpenter "52" alloy window ring using Thick Film Systems,Inc., 1162 silica seal glass paste which is fused ss frit seal. The sealis made by heating the combination to 440 degrees centigrade in anitrogen atmosphere which eliminates the need for relapping the opticalblank 40 when the heating is done in an air furnace. The oxidizingenvironment requires that the zinc selenide be relapped prior to havinga mirrored coating applied thereto.

Turning to FIG. 5, an alternate embodiment of a Waveguide laser inaccordance with FIGS. 1 through 4 is illustrated in a sectional viewsimilar to that shown in FIG. 4 except that special electrode postutilizes a different design from those shown in FIGS. 1 through 4. Theceramic Waveguide 11 has a pair of elongated slots 74 cut in each sidethereof, on each side of a pair of Waveguide bores 73. The bottom of theslots 74 have been metallized as before, but a special post clip 75 hasbeen shaped with a center arcuate portion 76 and shaped to follow thesides of the Waveguide slots 74. The electrode 75 can be snapped intoplaces in the slots 74 and welded or brazed as in the prior embodimentto the metallized surface.

Turning to FIG. 6, a flow diagram of the process of manufacturing alaser in accordance with FIGS. 1 through 5 is illustrated. A windowblank 85 is selected of a zinc selenide for forming the laser mirror ateach end of the laser. A window ring is made of carpenter metal havingan opening therein in the step 86 and the window blank 85 is placedtherein with a glass paste therebetween to form a glass frit seal whenthe window blank and window ring are heated with the glass paste to 450degrees centigrade at step 87. The attached window blank and window ringforming the zinc selenide, laser optics are then assembled to theoptical assemble for each end of the laser. A ceramic Waveguide isfabricated of almina with metallization of the ends at 1650 degreescentigrade in step 89 and is attached to the laser optic assembly and asoft seal laser dynamic test is performed in step 90. The vacuumenvelope parts 91 can be attached to the laser assembly which can becleaned and fired at 1,000 degrees centigrade in step 92 with the mirrorassembly 93 removed and put through helium nitrogen static alignment.Step 94: The laser assembly can then have the body assembled with agold/copper braze at 1,020 degrees centigrade at step 95 and the laserassembly heli-arc welded to the end of the Waveguide in step 96. Thisprovides for the kovar attachment cup of the laser optic assembly to beattached to the metallized Waveguide ends. A leak test is then performedin step 97 having a ten to the minus 9 CC per second test. The laserassembly is then vacuumed baked at 300 degrees centigrade forfourty-eight hours at step 98. This can be accomplished with the presentlaser because of the frit seals in step 87 which can withstandtemperatres up to 450 degrees without melting or otherwise leaking. Thisallows water and any impurities in the laser system to be cleaned outthereof so that the laser can be filled with a CO₂ gas mixture 100through the back filled pinch-off tube in step 99 and then the pinch-offtube pinched off. The window blank is selected of a zinc selenide mirrorwhich has a glass frit seal of similar expansion attached to a carpenteralloy 52 disk which also has a similar expansion rate to the zincselenide mirrors. These are, in turn, welded to the kovar tube 37. Aclosed loop circulation of the CO₂ gas mixture is provided bymagnetically coupled pump attached to the gas ballast tank which in turnis connected by copper tubing and connectors to the wave guide opticalassemblies. An aluminum heat sink is attached to the ultimate laser butis not illustrated. High voltage power supply is connected to theelectrodes 21 through the anode and cathode posts 22.

It should be clear at this point that both a CO₂ wave guide laserapparatus has been illustrated along with the method of manufacturing awave guide laser apparatus. The laser includes all hard sealconstruction with a glass frit connection of the glass and gold brazealloy connection between the metal components to provide a laser whichcan have high temperature processing at 400 degree centigrade to driveout water and contaminates. But also, to provide for a laser with a longshelf life and long operating life. Gas circulation is within a sealedenvelope using a magnetic pump to stabilize the laser output. Invarstabilizing rods are utilized to match the expansion of the aluminaceramic Waveguide to reduce the temperature expansion but the laser usesan opposite expanding kovar expansion sleeve connected to the optics. Ahigh precision mirror adjustment is also provided with a flexure(bellows like) kovar member driven by threaded members driving the invarstabilizing rods to shift the optics. The use of all hard seals with noelastomer or epoxy seals allows for the high temperature processing alsohelps to provide for the ten year shelf life. The electrodes areexternal to the Waveguide bore and are placed in slots so that they aresturdy and avoid any sputtering in the laser.

However, the present invetion is not to be considered limited to theforms shown which are to be considered illustrative rather thanrestrictive.

I claim:
 1. A process of making a gas laser apparatus comprising thesteps of:selecting an optical mirror blank; making a metal mirror blanksupport ring having an opening therethrough for mounting the selectedmirror blank thereon and having a heat expansion rate generally matchingthe heat expansion rate of said optical mirror blank; applying a glassfrit material between the mirror blank and the mirror blank supportring, said glass frit having a heat expansion rate generally matchingthe heat expansion rate of said optical mirror blank and metal mirrorblank support ring; heating the mirror blank and the metal mirror blanksupport ring and glass paste material thereon to a temperature in excessof 400 degrees centigrade in a nitrogen atmosphere to seal said mirrorblank to said metal mirror blank support ring with a glass frit seal;attaching said mirror blank support ring having the mirror blankattached thereto to a temperature expansion sleeve of a gas laserassembly to thereby form a laser with a high temperature glass to metalseal for the mirrors in a gas laser; and vacuum baking the laserassembly at approximately 300 degrees centrigrade for at leasttwenty-four hours to remove moisture and impurities therefrom.
 2. Aprocess of making a gas laser apparatus in accordance with claim 1including the step of selecting a waveguide material and, opticalassembly support rods for a temperature expansion of approximately thesame expansion as the temperature expansion sleeve whereby oppositeexpansion of the temperature expansion sleeve with offset the ceramicwaveguide and optical assembly support rods temperature expansion.
 3. Aprocess of making a gas laser apparatus in accordance with claim 2including the steps of selecting a metal mirror blank supporting ring ofcarpenter's metal and an optical mirror blank of zinc selenide and asilica seal glass paste for heating to a temperature of approximately440 degrees centigrade and the welding the metal mirror blank supportingring to a kovar temperature expansion sleeve.